Rear suspension - help needed.

[Mr A]

Here is the problem.

I have changed the wheel set up on my 94 Miata 1.8i. The new front tires are 195/60/14 and work just fine. The rears are now 205/60/14 with a 4 3/4" backspace, so they hang out the body considerably (not the issue cause that's what I need for the new look). The car is due to get a new fiberglass body kit on top too, which will make it much heavier and the rubber already sits on the wheel well.

So the problem is this - I want to change the rear suspension set up to take account of the new weight and need more height to stop the tires getting shredded by the arches when in motion. So any ideas?

1: What needs to be considered when changing the component shocks/springs (weight of vehicle? strain on chassis? etc).

2: I need the car to be UK legal and drivable (but not really concerned with performance as this will be a more 'fun' project than daily driver and driven a few 1,000 miles per year.

Any help would be appreciated as I know nothing about cars and am trying to research this before starting the job.